1. Field of the Invention
The present invention relates in general to a system for creating accurately colored images of land areas based on limited spectral radiation data gathered by spacecraft orbiting and scanning the land, and in particular, the red and near infrared images available from present weather satellites.
2. Description of Related Art
Orbiting sensors and other imaging devices have generated large amounts of information about the Earth. Unfortunately a realistic, cloudless, true color global image of the entire Earth with sufficient resolution to identify many land features has not been produced. Satellites and aircraft have taken many high resolution photographs of portions of the Earth but such photographs are not extensive enough or sufficiently coherent to assemble an unobscured and undistorted composite Earth image. Many factors have conspired to prevent construction of a realistic composite color photographic image of the Earth. For example, much of the Earth's surface is usually obscured by clouds or haze. Even clear atmosphere increasingly obscures and distorts color and detail as the photographic view is directed away from vertical.
Various non-photographic imaging sensors have been deployed in space craft for monitoring weather and earth resources, but most such sensors provide only low resolution images or fail to scan the entire Earth. The National Oceanic and Atmospheric Administration's (NOAA) Television Infrared Orbiting Satellites (TIROS) include Advanced Very High Resolution Radiometry (AVHRR) packages scanning the entire Earth approximately twice a day to produce five sensor data streams ("Channels") imaging the earth with a resolution of about 1 kilometer. The Channel One scanner senses the intensity of light radiation in the visible red spectrum between 580 and 680 nanometers in wavelength. The Channel Two scanner covers the "near-infrared" spectrum of radiation with wavelengths between 725 and 1,100 nanometers. Channels Three to Five sense infrared portions of the spectrum. Thus the five channels carry data streams representing five different images of the Earth sampling separate parts of the red through infrared portion of the radiation spectrum. The images are pixel-based and relatively high in resolution, with each pixel of the image representing the measured radiation intensity at a spot on the Earth about 1 kilometer in diameter.
The AVHRR system was designed primarily to monitor weather and temperature patterns in the oceans and the atmosphere, not to image the land surface in the visible spectrum. But the data it produces has been used for other purposes, for example to measure the amount of land-based vegetation. Since green plants absorb most red light while reflecting near-infrared light, the Normalized Difference Vegetation Index (NDVI), a measure of the difference between the red light intensity of Channel One and near-infrared intensity of Channel Two, has been used to image vegetation density. A high NDVI value indicates productive vegetation while a low NDVI value indicates bare soil, clouds, water or ice. However colors used to distinguish vegetated areas of the Earth from non-vegetated areas are arbitrary and do not represent the true colors of such areas.
There have been attempts to use the AVHRR data to produce color images of land portions of the Earth, but such attempts have been relatively unsuccessful, having failed to accurately model the relationship between land color and available AVHRR data. It is common to create a pixel-based image where each pixel is formed by mixing various levels of red, green and blue (RGB) components. One might use the red spectrum data from Channel One to control the intensity of the red component, NDVI to control the intensity of the green component, and various combinations of AVHRR data to control the intensity of the blue component. However, while the Channel One data accurately indicates the red component, the green component is only loosely related to NDVI and the blue component has not been accurately predicted by any previously known combinations of AVHRR channel data. Thus previous attempts to color images produced based on AVHRR data have provided only poor approximations of the true colors of the Earth. Such failures arise because there is no fixed, single-valued relationship between the limited spectrum radiation represented by AVHRR data and visible spectrum color. Except for its visible red spectrum portion, such radiation does not arise as a result of the land's color but from other characteristics of the land. Differently colored lands can and frequently do produce similar AVHRR radiation patterns.
What is needed is a method for producing high resolution images depicting true colors of land portions of the Earth's surface as accurately as possible from the limited spectrum scan AVHRR data currently provided by the NOAA.